Aluminosilicate omega-type zeolitic molecular sieves have a composition generally expressed in terms of mole ratios of oxides as follows: EQU 1.0.+-.0.5M.sub.2/n O:Al.sub.2 O.sub.3 :3-20SiO.sub.2 :0-20H.sub.2 0
wherein M is at least one cation in oxidation state n. Their structure is a framework of gmelinite cages in two networks of non-intersecting channels--a series of main channels formed of 12-sided rings that run parallel to the c axis of the unit cell; and a series of secondary channels in the a-b plane that are inaccessible to the main channels. These materials exhibit characteristic X-ray diffraction patterns and may be useful as adsorbents and catalysts.
The most common and best characterized omega-type zeolites are zeolite omega and zeolite ZSM-4. Both are prepared from alkaline alumina-silica mixtures containing templating agents. As detailed in U.S. Pat. No. 4,241,036 the favored template for zeolite omega is an alkylammonium cation, preferably tetramethylammonium (TMA) cation. For ZSM-4 several templating agents have been reported, including TMA salts (British Pat. No. 1,297,256), other alkylammonium salts (British Pat. No. 1,365,318), pyrrolidine and chlorine salts (U.S. Pat. No. 4,021,447), diazobicyclooctane and triethylenediamine (U.S. Pat. No. 4,331,643) and various oxygen-containing and oxygen-nitrogen-containing organic compounds (U.S. Pat. No. 4,377,502). Ion-exchange of the as-synthesized or template-containing zeolite is incomplete. Only the metal ions contained in the main channels can be removed in the manner. The template cations are trapped in the gmelinite cages and can only be removed by vigorous methods such as calcination and oxidation. When the oxidized or calcined form is exchanged with ammonium ion and then treated with ammonium fluorosilicate (AFS) to replace zeolitic aluminum with silicon according to the method described in U.S. Pat. No. 4,503,023, the product retains no more than 30% crystallinity.
The principal object of this invention is the preparation of an omega-type zeolite without the use of a templating agent. It is a further object to prepare an omega-type zeolite which is totally ion-exchangeable in its as-synthesized, uncalcined form. It is an additional object to provide a novel omega-type zeolite, LZ-202, which, when treated with fluorosilicate salts, is converted to a substantially crystalline, silicon-enriched zeolitic material.